1. Field of the Invention
The invention relates to a method of designing a layout of a semiconductor integrated circuit, a program for causing a computer to carry out the method, a method of fabricating a semiconductor integrated circuit, and an apparatus for designing a layout of a semiconductor integrated circuit.
2. Description of the Related Art
An internal circuit in a semiconductor integrated circuit such as LSI (Large Scale Integration) is accompanied with a problem that electrical noises generated when the internal circuit operates cause fluctuation in delay and malfunction.
For instance, such electrical noises are caused by fluctuation in a voltage of a power-source line, generated when a transistor is turned on or off.
Such electrical noises can be reduced by means of an on-chip capacitor arranged in LSI.
For instance, Japanese Patent Application Publication No. 11-168177 (June, 1999) has suggested a method of arranging a functional block and an on-chip capacitor in LSI.
FIG. 1 is a flow chart showing steps to be carried out in the method.
As illustrated in FIG. 1, first, functional blocks are placed so as to satisfy requirements such as connection in a net list, a given delay, and a density of functional blocks, in step S201.
Then, there are fabricated on-chip capacitors each having a size equal to or smaller than a size of each of spaces where functional blocks were not arranged, and the thus fabricated on-chip capacitors are arranged in the spaces, in step S202.
Then, the functional blocks are routed, that is, electrically connected to one another through wires, in step S203.
In the method shown in FIG. 1, a layout of the functional blocks is designed (step S201) before a layout of the on-chip capacitors is designed (step S202).
As described in the above-mentioned Publication, an on-chip capacitor is preferably disposed in the vicinity of a functional block in order to reduce electrical noises.
However, the method suggested in the above-mentioned Publication is accompanied with a problem that since an on-chip capacitor is disposed in a space where functional blocks have not been arranged, it is not always for an on-chip capacitor to be disposed in the vicinity of a functional block, resulting in insufficient reduction in electrical noises.
Furthermore, the method suggested in the above-mentioned Publication is accompanied further with a problem that functional blocks are arranged in excessively high integration. In the case that a layout of on-chip capacitors is designed after a layout of functional blocks has been determined in excessively high integration, it would not be possible to arrange on-chip capacitors with sufficient space being left between adjacent functional blocks. This results in insufficient reduction in electrical noises.
Japanese Patent Application Publication No. 2001-351985 has suggested a method of designing a layout of a semiconductor integrated circuit. In the method, there are prepared a first library including no bypass capacitor, a second library including a bypass capacitor having a small capacity, and a third library including a bypass capacitor having a high capacity, for one logic including data indicative of a size of a transistor. The third library is used for a logic having a high toggle rate, and the second library is used for a logic having a small toggle rate.
Japanese Patent Application Publication No. 2004-86881 has suggested an apparatus for designing a semiconductor integrated circuit. The apparatus receives data relating to a gate level logic circuit of circuit blocks in LSI chip, data relating to standard cell library, and data relating to a package, and analyzes noises of the LSI chip in accordance with the received data. If noises are within an allowable range, the apparatus continues analyzing noises. If noises are without an allowable range, the apparatus selects one of logic gates in circuit blocks, and add a bypass capacitor to the selected logic gate.